Standard voltage reference cells (herein cells) regardless of the care used in production vary in voltage output when measured in the microvolt and nanovolt range. For applications requiring voltage accuracy and for use in standards labs groups of cells are intercompared to detect variances in voltage between the cells.
Under the Group Voltage Measurement Assurance Program sponsored by the National Bureau of Standards (NBS), individual cells in groups of four or six cells are intercompared with individual cells in other groups. By intercomparison is meant determining the voltage potential difference between two cells. NBS has recommended a method of intercomparing cells (see Eicke, Woodward G. and Auxier, Laurel M., Regional Maintenance of the Volt Using NBS Volt Transfer Techniques, IEEE TRANSACTIONS, Vol IM-23, No. 4, December 1974, pp. 290-294) by measuring the voltage difference between pairs of cells in a statistically balanced design. For instance, one possible design for comparing two groups of four cells is shown in Table I below:
TABLE I ______________________________________ NO. A SIDE B SIDE ______________________________________ 1 Group 1, Cell 1 (vs) Group 2, Cell, 1 2 Group 1, Cell 1 (vs) Group 2, Cell, 3 3 Group 1, Cell 3 (vs) Group 2, Cell, 3 4 Group 1, Cell 3 (vs) Group 2, Cell, 1 5 Group 1, Cell 2 (vs) Group 2, Cell, 2 6 Group 1, Cell 2 (vs) Group 2, Cell, 4 7 Group 1, Cell 4 (vs) Group 2, Cell, 4 8 Group 1, Cell 4 (vs) Group 2, Cell, 2 9 Group 2, Cell 2 (vs) Group 1, Cell, 1 10 Group 2, Cell 2 (vs) Group 1, Cell, 3 11 Group 2, Cell 4 (vs) Group 1, Cell, 3 12 Group 2, Cell 4 (vs) Group 1, Cell, 1 13 Group 2, Cell 1 (vs) Group 1, Cell, 2 14 Group 2, Cell 1 (vs) Group 1, Cell, 4 15 Group 2, Cell 3 (vs) Group 1, Cell, 4 16 Group 2, Cell 3 (vs) Group 1, Cell, 2 ______________________________________
As can be seen from Table I the eight cells (two groups of four) are compared using 16 measurements of voltage differences. Note also that each cell appears on the A side twice and on the B side twice.
When all cell comparisons are complete, a least squares fit of the values of the voltage difference between the cells can be computed. The values from the least squares fit can then be compared to the values measured and a standard deviation can be calculated.
Traditionally, voltage measurements between cells has been done by operators manually connecting cells together and detecting values. However, the amount of comparisons (e.g. 32 comparisons for comparing four groups of four cells), results in a great amount of time being spent by operators in performance of the cell measurements. Additionally, operator technique can affect the measured value resulting in different values being detected by different operators. Furthermore, there is a heightened danger of cell damage when measurement leads on cells are moved manually by operators. Commercially available cell scanners have proved to be either too expensive or not able to measure voltage values to sufficient levels of precision.